Generally, it is preferable, for a truck or other large-sized transport vehicle traveling on unpaved rough roads more often than an ordinary passenger car, that an engine sucks in not air near the ground containing plenty of dust but clean air at a sufficient height above the ground. To eliminate a concern of sucking in splashing rainwater or snow together near the ground, it is preferable to ensure that the engine sucks in the air only at the sufficient height above the ground.
Therefore, as exemplarily shown in FIG. 1, a large-sized transport vehicle has an air cleaner 1 with a vertical air-cleaner inlet duct 2 connected thereto. An air intake duct 4 fixed to a rear of a cab 3 is connected via a flexible boot 5 to the inlet duct 2 so that fresh air may be sucked in through an air intake 6 opened on an upper portion of the duct 4.
The inventors examined a flow velocity (flow rate) of fresh air sucked in through such air intake duct 4 to find out that no matter how large an opening area of the air intake 6 is, fresh air is sucked in one-sidely through a lower portion of the intake 6 and hardly through an upper portion of the intake 6. It is feared that the one-sided suction of fresh air through the lower portion of the air intake 6 may increase the flow velocity, leading to promoted suction of rain, snow and/or dust through the lower portion of the intake 6.
Thus, in order to uniform a flow velocity distribution of fresh air sucked in through the air intake 6, it has been considered to divide an interior of the air intake duct 4 into a plurality of channels to form a multi-cell structure. Conventionally, the division of the interior of the air intake duct 4 into the plural channels is made such that a portion of the air intake duct 4 is pinched by and crushed between dies (not shown) to form a partition wall 7 in a blow molding process which provides divided channels A and B, as shown in section in FIG. 2.
A state-of-art technology relating to ordinary air intake ducts is disclosed, for example, in Patent Literature 1 mentioned below.